1. Field of the Invention
The present invention relates to a method of determining a heat treatment to apply to a structural member, in particular to modify the deformation behavior of the member. The invention also relates to impact protection members when heat treated, so as to modify their deformation behavior.
In a vehicle crash situation, energy absorbing structures experience large variations in the local stresses and strains. While some regions of a member need to withstand high stresses, other areas are required to accommodate high strains in order to avoid problems of brittle fracture. The particular alloy, for example an aluminium alloy, that is chosen for the structure should be selected with both of these criteria in mind.
Increasing the gauge thickness in the product will normally satisfy the stress requirements but not without imposing a weight limitation. Quite often, high strength alloys are not suitable in crash absorbing products due to their limited ability to withstand high strains in exposed regions of the part. Traditionally, the solution to this problem is to select a more ductile alloy with less yield strength. This again brings back the problem of insufficient strength of the structure, which must be compensated for by increasing the gauge thickness.
When designing automotive parts, due attention must be given to the deformation behavior of the structure when it is submitted to high stresses or strains, for example during an accident. In particular, the front and rear bumper beams represent particularly exposed regions, and large efforts are put in controlling the deformation behavior of these regions. The more energy that can be absorbed in the outer part of the structure, the less damage is caused to the remaining internal part of the structure of the vehicle.
When constructing bumper beams, different methods are used in order to control the deformation behavior. For example, the use of tilted partitions and non-uniform plate thicknesses makes it possible to attain favorable deformation behaviors. Such physical geometrical features are, however, demanding and expensive to produce. Great effort is therefore directed at the construction of the simplest possible structure, which also exhibits the best possible deformation behavior.
2. Description of the Related Art
One example of a composite bumper of this kind is described in U.S. Pat. No. 4,252,355 which discloses a bumper section which is integrally molded with lugs. The bumper section and lugs each contain elongated reinforcement fibers. The energy of a crash is absorbed, at least in part, by the deformation of the lugs.
To enhance the effectiveness of bumper beams, crash boxes have been introduced between the inner vehicle structure and the external structure. When the forces on a crash box exceed certain critical values, the crash box deforms and thereby absorbs energy.
As is the case for bumper beams, the deformation behavior of a crash box during an impact is critical. If the crash box resists too high a stress, its impact absorbing capacity will occur after damage is done to the vehicle structure. If there are protrusions or holes in the side walls of the crash box, they will generate local zones of high stress concentration. The possibility of local crack propagation in these zones is increased due to the stress concentration. Cracking changes the deformation behavior of the crash box, and the ability to absorb energy by deformation is reduced when extensive cracking occurs.
One method of improving the ductility in a bearing member for a vehicle is disclosed in U.S. Pat. No. 5,492,207. The bearing member is subjected to a heat treatment to change the mechanical properties of the material from which the bearing member is constructed. As a result, the deformation behavior of the material is improved.
There is, therefore, a desire to provide an improved method of modifying the deformation behavior in such structural members, the goal being to achieve a fast and low cost method that is versatile in its applicability to different types of structural members.